This invention relates to microwave oscillator circuits and more particularly to a system and method for measuring resonance phenomena in such circuits.
Radar stable local oscillators are becoming increasingly sophisticated with a continuing trend toward better short term stability coupled with fast wide range frequency agility. Recently developed high overtone bulk acoustic resonator (HBAR) technology has permitted direct stabilization of microwave oscillators with significantly improved short term frequency stability. This may be achieved by using a high Q HBAR resonance to filter the output of a microwave voltage control oscillator (VCO), with the filtered output furnishing a reference for a phase locked loop containing the VCO. When frequency agility is required, the system is configured such that the VCO is prepositioned close enough to an HBAR resonance such that the stabilization feedback loop can complete lock-up. Because a typical HBAR may exhibit as many as 200 resonances spaced at say 5 megahertz intervals, it is important to know which of these the loop has locked onto, particularly in systems requiring rapid frequency agility. Conventional methods of heterodyning down and frequency counting require a significant body of electronics and demand good local oscillator stability. Such a technique is cumbersome and relatively slow, hence not well suited for use with fast frequency agile sources. Therefore, there is a need for a means of rapid identification of the controlling HBAR resonance, preferably with some indication of microwave signal quality as lock-up is achieved, in a manner which has minimal impact on the microwave circuitry and no adverse effects on circuit operation.